Field
The disclosed concept relates generally to electrical switching apparatus and, more particularly, to electrical switching apparatus, such as circuit breakers. The disclosed concept also relates to thermal trip assemblies for electrical switching apparatus.
Background Information
Electrical switching apparatus, such as circuit breakers, are known to be employed in electrical systems to protect a portion of a circuit during certain predetermined conditions such as, for example, in response to a trip condition (e.g., without limitation, an overcurrent condition; a relatively high level short circuit or fault condition; a ground fault or arc fault condition).
Relatively small molded case circuit breakers, for example, typically include one or more trip devices such as a magnetic trip assembly, a thermal trip assembly, etc., each of which cooperates with an operating mechanism that is configured to move at least one pair of separable contacts of the circuit breaker between an ON condition and a TRIPPED or an OFF condition when one or more of the predetermined conditions in the protected circuit are met. Each pair of separable contacts includes a stationary contact and a movable contact disposed on a corresponding movable (e.g., pivotable) contact arm. The operating mechanism is typically electrically connected to the thermal trip assembly by a number of flexible conductors or shunts.
Thermal trip assemblies typically include a bimetal and a number of heater elements. In operation, for example in response to an overload condition, electric current drawn by the load heats the heater elements which, in turn, heat the bimetal causing it to move (e.g., bend) and thereby, directly or indirectly, cooperate with a trip bar of the operating mechanism causing the trip bar to move (e.g., pivot) thereby pivoting the attached movable contact arm(s) and tripping open (e.g., separating) the separable contacts of the circuit breaker and interrupting the flow of electric current. Thus, the thermal trip assembly functions to provide a thermal trip response that is directly related to the magnitude of current drawn by the load. While such trip devices have been generally effective for their intended purposes, they have not been without limitations. For example, resistive forces from the shunt(s) on the bimetal can cause undesirable issues with respect to tripping of the circuit breaker. Potential for excessive heat to be generated by the bimetal can also be a cause for concern. Further, various factors, such as limited available space within the circuit breaker housing, present design challenges with respect to the structure, location and function or operation of the shunt(s), load conductor(s) and/or other components.
There is, therefore, room for improvement in electrical switching apparatus and in thermal trip assemblies therefor.